High-directivity directional coupler, and related methods and systems

ABSTRACT

Embodiments of the disclosure relate to a high-directivity directional coupler, and related methods and systems. The high-directivity directional coupler includes a first microstrip and a second microstrip disposed parallel to the first microstrip. The high-directivity directional coupler inherently generates an even mode current and an odd mode current in the second microstrip. The second microstrip provides a linear forward path for conveying the even mode current and a non-linear return path for conveying the odd mode current. The non-linear return path is longer than the linear forward path to compensate for phase velocity difference between the even mode current and the odd mode current. As a result, the odd mode current and the even mode current in the second microstrip can destructively cancel out each other, thus rendering high-directivity without compromising other performance aspects of the high-directivity directional coupler.

BACKGROUND

The disclosure relates generally to a directional coupler, and moreparticularly to a high-directivity directional coupler that can beprovided in a remote unit in a wireless distribution system (WDS).

Wireless customers are increasingly demanding digital data services,such as streaming video signals. At the same time, some wirelesscustomers use their wireless communications devices in areas that arepoorly serviced by conventional cellular networks, such as insidecertain buildings or areas where there is little cellular coverage. Oneresponse to the intersection of these two concerns has been the use ofdistributed antenna systems (DASs). DASs include remote units configuredto receive and transmit communications signals to client devices withinthe antenna range of the remote units. DASs can be particularly usefulwhen deployed inside buildings or other indoor environments where thewireless communications devices may not otherwise be able to effectivelyreceive radio frequency (RF) signals from a source.

In this regard, FIG. 1 illustrates distribution of communicationsservices to remote coverage areas 100(1)-100(N) of a wirelessdistribution system (WDS) provided in the form of a DAS 102, wherein ‘N’is the number of remote coverage areas. These communications servicescan include cellular services, wireless services, such RF identification(RFID) tracking, Wireless Fidelity (Wi-Fi), local area network (LAN),and wireless LAN (WLAN), wireless solutions (Bluetooth, Wi-Fi GlobalPositioning System (GPS) signal-based, and others) for location-basedservices, and combinations thereof, as examples. The remote coverageareas 100(1)-100(N) may be remotely located. In this regard, the remotecoverage areas 100(1)-100(N) are created by and centered on remote units104(1)-104(N) connected to a central unit 106 (e.g., a head-endequipment, a head-end controller, or a head-end unit). The central unit106 may be communicatively coupled to a signal source 108, for example abase transceiver station (BTS) or a baseband unit (BBU). In this regard,the central unit 106 receives downlink communications signals 110D fromthe signal source 108 to be distributed to the remote units104(1)-104(N). The remote units 104(1)-104(N) are configured to receivethe downlink communications signals 110D from the central unit 106 overa communications medium 112 to be distributed to the respective remotecoverage areas 100(1)-100(N) of the remote units 104(1)-104(N). Each ofthe remote units 104(1)-104(N) may include an RF transmitter/receiverand a respective antenna 114(1)-114(N) operably connected to the RFtransmitter/receiver to wirelessly distribute the communicationsservices to client devices 116 within the respective remote coverageareas 100(1)-100(N). The remote units 104(1)-104(N) are also configuredto receive uplink communications signals 110U from the client devices116 in the respective remote coverage areas 100(1)-100(N) to bedistributed to the signal source 108. The size of each of the remotecoverage areas 100(1)-100(N) is determined by the amount of RF powertransmitted by the respective remote units 104(1)-104(N), receiversensitivity, antenna gain, and RF environment, as well as by RFtransmitter/receiver sensitivity of the client devices 116. The clientdevices 116 usually have a fixed maximum RF receiver sensitivity, sothat the above-mentioned properties of the remote units 104(1)-104(N)mainly determine the size of the respective remote coverage areas100(1)-100(N).

The remote units 104(1)-104(N) may include power amplifiers118(1)-118(N), respectively, for amplifying the downlink communicationssignals 110D to specific power levels before transmitting the downlinkcommunications signals 110D to the client devices 116. As such, it maybe necessary to calibrate the power amplifiers 118(1)-118(N) duringinstallation and/or a configuration phase of the DAS 102 to ensure thatthe downlink communications signals 110D are amplified properly. In anon-limiting example, it is possible to calibrate each of the poweramplifiers 118(1)-118(N) using a directional coupler that includes aninput port, an output port, a coupled port, and an isolated port.

To calibrate a selected power amplifier among the power amplifiers118(1)-118(N), the input port of the directional coupler may be coupledto the selected power amplifier to receive a respective downlinkcommunications signal outputted by the selected power amplifier. Testequipment, such as a power detector, may be coupled to the coupled portof the directional coupler to receive a coupled signal that isproportional to the respective downlink communications signal receivedat the input port. Thus, by measuring a power level of the coupledsignal, it may be possible to determine a corresponding power level ofthe downlink communications signal received at the input port.Accordingly, it may be possible to configure the selected poweramplifier to output the respective downlink communications signal at adesired power. In this regard, performance merits of the directionalcoupler, such as insertion loss, coupling ripple, and directivity, candetermine the result of power amplifier calibration, and thus theperformance of the remote units 104(1)-104(N).

No admission is made that any reference cited herein constitutes priorart. Applicant expressly reserves the right to challenge the accuracyand pertinency of any cited documents.

SUMMARY

Embodiments of the disclosure relate to a high-directivity directionalcoupler, and related methods and systems. In a non-limiting example,directivity is an indication of how well a directional coupler candiscriminate between forward and return signals. In examples discussedherein, a high-directivity directional coupler is a directional couplercapable of rendering more than fifteen decibels (15 dB) in directivity.The high-directivity directional coupler includes a first microstrip anda second microstrip disposed parallel to the first microstrip. Thehigh-directivity directional coupler inherently generates an even modecurrent and an odd mode current in the second microstrip. The secondmicrostrip provides a linear forward path for conveying the even modecurrent and a non-linear return path for conveying the odd mode current.The non-linear return path is longer than the linear forward path tocompensate for phase velocity difference between the even mode currentand the odd mode current. As a result, the odd mode current and the evenmode current in the second microstrip can destructively cancel out eachother, thus rendering high-directivity without compromising otherperformance aspects of the high-directivity directional coupler.

In this regard, in one aspect, a high-directivity directional coupler isprovided. The high-directivity directional coupler includes a substrate.The high-directivity directional coupler also includes a ground planedisposed underneath the substrate. The high-directivity directionalcoupler also includes an input port configured to receive an inputsignal. The high-directivity directional coupler also includes an outputport configured to output the received input signal as an output signal.The high-directivity directional coupler also includes a coupled portconfigured to output a coupled signal proportional to the input signal.The high-directivity directional coupler also includes an isolated portconfigured to provide isolation to the input signal and the coupledsignal. The high-directivity directional coupler also includes a firstmicrostrip disposed above the substrate. The first microstrip isconfigured to convey the input signal from the input port to the outputport. The high-directivity directional coupler also includes a secondmicrostrip disposed above the substrate parallel to the firstmicrostrip. The second microstrip is configured to provide a linearforward path for conveying an even mode current from the coupled port tothe ground plane. The second microstrip is also configured to provide anon-linear return path longer than the linear forward path for conveyingan odd mode current in an opposite direction from the even mode current.

In another aspect, a method for fabricating a high-directivitydirectional coupler is provided. The method includes providing asubstrate. The method also includes providing a ground plane underneaththe substrate. The method also includes providing an input portconfigured to receive an input signal. The method also includesproviding an output port configured to output the received input signalas an output signal. The method also includes providing a coupled portconfigured to output a coupled signal proportional to the input signal.The method also includes providing an isolated port configured toprovide isolation to the input signal and the coupled signal. The methodalso includes disposing a first microstrip above the substrate. Thefirst microstrip is configured to convey the input signal from the inputport to the output port. The method also includes disposing a secondmicrostrip above the substrate parallel to the first microstrip. Thesecond microstrip is configured to provide a linear forward path forconveying an even mode current from the coupled port to the groundplane. The second microstrip is also configured to provide a non-linearreturn path longer than the linear forward path for conveying an oddmode current in an opposite direction from the even mode current.

In another aspect, a wireless distribution system (WDS) is provided. TheWDS includes a plurality of remote units. The plurality of remote unitsis configured to receive and convert a plurality of downlink digitalcommunications signals into a plurality of downlink radio frequency (RF)communications signals for distribution to client devices. The pluralityof remote units is also configured to receive a plurality of uplink RFcommunications signals from the client devices and convert the pluralityof uplink RF communications signals into a plurality of uplink digitalcommunications signals. The WDS also includes a central unit. Thecentral unit is configured to distribute the plurality of downlinkdigital communications signals to the plurality of remote units. Thecentral unit is also configured to receive the plurality of uplinkdigital communications signals from the plurality of remote units. Atleast one selected remote unit among the plurality of remote unitsincludes a high-directivity directional coupler. The high-directivitydirectional coupler includes a substrate. The high-directivitydirectional coupler also includes a ground plane disposed underneath thesubstrate. The high-directivity directional coupler also includes aninput port configured to receive an input signal. The high-directivitydirectional coupler also includes an output port configured to outputthe received input signal as an output signal. The high-directivitydirectional coupler also includes a coupled port configured to output acoupled signal proportional to the input signal. The high-directivitydirectional coupler also includes an isolated port configured to provideisolation to the input signal and the coupled signal. Thehigh-directivity directional coupler also includes a first microstripdisposed above the substrate. The first microstrip is configured toconvey the input signal from the input port to the output port. Thehigh-directivity directional coupler also includes a second microstripdisposed above the substrate parallel to the first microstrip. Thesecond microstrip is configured to provide a linear forward path forconveying an even mode current from the coupled port to the groundplane. The second microstrip is also configured to provide a non-linearreturn path longer than the linear forward path for conveying an oddmode current in an opposite direction from the even mode current.

Additional features and advantages will be set forth in the detaileddescription which follows and, in part, will be readily apparent tothose skilled in the art from the description or recognized bypracticing the embodiments as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary and are intendedto provide an overview or framework to understand the nature andcharacter of the claims.

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiment(s), and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary wireless distributionsystem (WDS), which may be a distributed antenna system (DAS) forexample;

FIG. 2A is a schematic diagram of an exemplary directional coupler;

FIG. 2B is a schematic diagram providing an exemplary illustration ofhow a forward signal and a reflected signal can impact directivity ofthe directional coupler of FIG. 2A;

FIG. 2C is a schematic diagram providing an exemplary illustration ofeven mode currents that are inherent to the directional coupler of FIG.2A;

FIG. 2D is a schematic diagram providing an exemplary illustration ofodd mode currents that are inherent to the directional coupler of FIG.2A;

FIG. 2E is a graph providing an exemplary illustration of variouscoupling ripples in the directional coupler of FIG. 2A;

FIG. 2F is a graph providing an exemplary illustration of directivity ofthe directional coupler of FIG. 2A;

FIG. 3A is a schematic diagram of an exemplary high-directivitydirectional coupler that can provide improved directivity withoutcompromising other performance measures (e.g., coupling ripple) in thehigh-directivity directional coupler;

FIG. 3B is a schematic diagram providing an exemplary top view of thehigh-directivity directional coupler of FIG. 3A;

FIG. 4 is a flowchart of an exemplary process for fabricating thehigh-directivity directional coupler of FIG. 3A;

FIG. 5A is a graph providing an exemplary illustration of variouscoupling ripples in the high-directivity directional coupler of FIG. 3A;

FIG. 5B is a graph providing an exemplary illustration of higherdirectivity in the high-directivity directional coupler of FIG. 3Acompared to the directional coupler of FIG. 2A;

FIG. 6 is a schematic diagram of an exemplary radio frequency (RF) powermanagement circuit in which the high-directivity directional coupler ofFIG. 3A is configured to provide an RF feedback signal from a poweramplifier to a digital pre-distortion (DPD) circuit to help improvelinearity of the power amplifier;

FIG. 7 is a schematic diagram an exemplary WDS provided in the form ofan optical fiber-based WDS in which at least one selected remote unitamong a plurality of remote units can employ the RF power managementcircuit of FIG. 6 for improving linearity of a power amplifier in theselected remote unit; and

FIG. 8 is a partial schematic cut-away diagram of an exemplary buildinginfrastructure in which a WDS, such as the WDS of FIG. 7, includes aremote unit that employs the high-directivity directional coupler ofFIG. 3A to provide linearity improvement in a power amplifier in theremote unit.

DETAILED DESCRIPTION

Embodiments of the disclosure relate to a high-directivity directionalcoupler, and related methods and systems. In a non-limiting example,directivity is an indication of how well a directional coupler candiscriminate between forward and return signals. In examples discussedherein, a high-directivity directional coupler is a directional couplercapable of rendering more than fifteen decibels (15 dB) in directivity.The high-directivity directional coupler includes a first microstrip anda second microstrip disposed parallel to the first microstrip. Thehigh-directivity directional coupler inherently generates an even modecurrent and an odd mode current in the second microstrip. The secondmicrostrip provides a linear forward path for conveying the even modecurrent and a non-linear return path for conveying the odd mode current.The non-linear return path is longer than the linear forward path tocompensate for phase velocity difference between the even mode currentand the odd mode current. As a result, the odd mode current and the evenmode current in the second microstrip can destructively cancel out eachother, thus rendering high-directivity without compromising otherperformance aspects of the high-directivity directional coupler.

Before discussing exemplary aspects of a high-directivity directionalcoupler, and related methods and systems, an overview of a directionalcoupler and corresponding figure of merits are first provided withreferences to FIGS. 2A-2F. The discussion of specific exemplary aspectsof a high-directivity directional coupler, and related methods andsystems, starts below with reference to FIG. 3A.

In this regard, FIG. 2A is a schematic diagram of an exemplarydirectional coupler 200. The directional coupler 200 includes a firstmicrostrip 202 and a second microstrip 204 that are disposed in parallelon a top surface 206 of a substrate 208. The directional coupler 200also includes a ground plane 210 disposed on a bottom surface 212 of thesubstrate 208. In this regard, the ground plane 210 is disposed on anopposite side of the substrate 208 from the first microstrip 202 and thesecond microstrip 204. The directional coupler 200 includes an inputport (P1) 214, an output port (P2) 216, a coupled port (P3) 218, and anisolated port (P4) 220. In this regard, the directional coupler 200 isalso known as a four-port directional coupler. The first microstrip 202couples the input port (P1) 214 with the output port (P2) 216. Thesecond microstrip 204 couples the coupled port (P3) 218 with theisolated port (P4) 220.

The directional coupler 200 may receive a signal 222 at an input powerP_(I) via the input port (P1) 214 and output the signal 222 at an outputpower P_(O) via the output port (P2) 216. The directional coupler 200can output a coupled signal 224 at a coupled power P_(C) via the coupledport (P3) 218. The output power P_(O) and the coupled power P_(C) areboth proportional to the input power P_(I). The directional coupler 200is configured to split the input power P_(I) between the output powerP_(O) and the coupled power P_(C) in such a way that the output powerP_(O) is substantially higher than the coupled power P_(C). As such, itis possible to determine the input power P_(I) of the signal 222 at theinput port (P1) 214 by observing and/or measuring the coupled powerP_(C) at the coupled port (P3) 218 without significantly reducing theoutput power P_(O) of the signal 222 at the output port (P2) 216.

The input power P_(I) and the coupled power P_(C) determine a couplingfactor C_(3,1) between the input port (P1) 214 and the coupled port (P3)218. The input power P_(I) and the output power P_(O) determine aninsertion loss L_(2,1) between the output port (P2) 216 and the inputport (P1) 214. The coupling factor C_(3,1) and the insertion lossL_(2,1) are shown below in equations (Eq. 1) and (Eq. 2), respectively.Both the coupling factor C_(3,1) and the insertion loss L_(2,1) aremeasured in decibels (dB).C _(3,1)=10 log(P _(C) /P _(I))  (Eq. 1)L _(2,1)=10 log(P _(O) /P _(I))  (Eq. 2)

The coupling factor C_(3,1) and the insertion loss L_(2,1) are twoimportant figures of merit of the directional coupler 200. A highercoupling factor C_(3,1) means a higher coupling loss between the inputport (P1) 214 and the coupled port (P3) 214. A lower insertion lossL_(2,1) means higher power transfer from the input port (P1) 218 to theoutput port (P2) 216. The directional coupler 200 is required to provideas much isolation as possible between the isolated port (P4) 220 and theinput port (P1) 214. In a non-limiting example, when the directionalcoupler 200 is provided in a radio frequency (RF) system, the isolationport (P4) 220 is commonly coupled to a fifty ohms (50Ω) resistor thatabsorbs reflected signal from the output port (P2) 216 to the isolatedport (P4) 220. Accordingly, an isolated power P_(ISO) measured at theisolated port (P4) 220 should be as low as possible.

Another important figure of merit for the directional coupler 200 isdirectivity D. To help understand how directivity D is defined anddetermined, FIG. 2B is discussed next. In this regard, FIG. 2B is aschematic diagram providing an exemplary illustration of how a forwardsignal 226 and a reflected signal 228 can impact the directivity D ofthe directional coupler 200 of FIG. 2A. Common elements between FIGS. 2Aand 2B are shown therein with common element numbers and will not bere-described herein.

With reference to FIG. 2B, the input port (P1) 214 of the directionalcoupler 200 is coupled to a transmission line 230 having impedance Z₁,while the output port (P2) 216 of the directional coupler 200 is coupledto load impedance Z_(L). When the impedance Z₁ matches the loadimpedance Z_(L), the forward signal 226 will travel from the input port(P1) 214 to the output port (P2) 216 without being reflected. As aresult, the reflected signal 228 would be minimal. However, when theimpedance Z₁ does not match perfectly with the load impedance Z_(L), aportion of the forward signal 226 would be reflected as the reflectedsignal 228. As a result, a portion of the reflected signal 228 may beleaked to the coupled port (P3) 218 as a leaked signal 232.Consequently, when a detector 234 is coupled to the coupled port (P3)218 to observe and measure the coupled signal 224, the detector 234would instead receive a combination of the coupled signal 224 and theleaked signal 232, thus causing an inaccurate reading of the coupledsignal 224. In this regard, the directivity D of the directional coupler200 determines how well the directional coupler 200 discriminatesbetween the forward signal 226 and the reflected signal 228. Thedirectivity D may be determined based on the equation (Eq. 3) below.D=C _(3,1) +I _(3,2)  (Eq. 3)

In the equation (Eq. 3) above, C_(3,1) represents the coupling factorbetween the input port (P1) 214 and the coupled port (P3) 218 asdetermined based on Eq. 1, and I_(3,2) represents isolation between thecoupled port (P3) 218 and the output port (P2) 216. The isolationI_(3,2) indicates the ability of the directional coupler 200 to rejectand/or cancel out the reflected signal 228. The isolation I_(3,2) can bedetermined based on the output power P_(O) and the coupled power P_(C)as shown in the equation (Eq. 4) below. The isolation I_(3,2) is alsomeasured in dB.I _(3,2)=−10 log(P _(C) /P _(O))  (Eq. 4)

Thus, by substituting the coupling factor C_(3,1) and the isolationI_(3,2) of Eq. 3 with Eq. 1 and Eq. 4, respectively, Eq. 3 can berewritten as the equation (Eq. 5) below. The directivity D is alsomeasured in dB.D=10 log(P _(C) /P _(I))−10 log(P _(C) /P _(O))  (Eq. 5)

In this regard, the higher the directivity D, the less leaked signal 232at the coupled port (P3) 218, and thus a more accurate reading of thecoupled signal 224 can be obtained at the coupled port (P3) 218.

With reference back to FIG. 2A, when the signal 222 flows from the inputport (P1) 214 toward the output port (P2) 216 in the first microstrip202, the electrical coupling between the first microstrip 202 and thesecond microstrip 204 causes the coupled signal 224 to be outputted fromthe coupled port (P3) 218. The signal 222 inherently causes an even modecurrent 236 to flow from the coupled port (P3) 218 toward the isolatedport (P4) 220 in the second microstrip 204. In this regard, the evenmode current 236 is the result of an even mode excitation of the signal222 in the first microstrip 202. The even mode current 236 is associatedwith even mode characteristic impedance Z_(0e), which can be expressedby the equation (Eq. 6) below.

$\begin{matrix}{Z_{0\; e} = {Z_{0}\sqrt{\left( {1 + C} \right)/\left( {1 - C} \right)}}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

In the equation (Eq. 6) above, C (C≤1) represents a voltage-couplingfactor of the directional coupler 200, while Z₀ represents single-linecharacteristic impedance of the first microstrip 202 or the secondmicrostrip 204.

The signal 222 causes an electrical field 238 to be generated betweenthe output port (P2) 216 and the isolated port (P4) 220. As such, thesignal 222 also inherently causes an odd mode current 240 to flow fromthe isolated port (P4) 220 toward the coupled port (P3) 218 in thesecond microstrip 204. In this regard, the odd mode current 240 is theresult of an odd mode excitation of the signal 222. Hence, to be able toaccurately observe the coupled signal 224 at the coupled port (P3) 218,it is necessary for the even mode current 236 to destructively cancelout the odd mode current 240. The odd mode current 240 is associatedwith odd mode characteristic impedance Z_(0o), which can be expressed bythe equation (Eq. 7) below.

$\begin{matrix}{Z_{0o} = {Z_{0}\sqrt{\left( {1 - C} \right)/\left( {1 + C} \right)}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

In the equation (Eq. 7) above, C (C≤1) represents the voltage-couplingfactor of the directional coupler 200. Z₀ represents single-linecharacteristic impedance of the first microstrip 202 or the secondmicrostrip 204.

The even mode current 236 and the odd mode current 240 are importantcharacteristics inherent to a pair of coupled lines, such as the firstmicrostrip 202 and the second microstrip 204 in the directional coupler200. As such, FIGS. 2C and 2D are discussed next to help understand howthe even mode current 236 and the odd mode current 240 can potentiallyimpact the accuracy in measuring the coupled signal 224 at the coupledport (P3) 218.

In this regard, FIG. 2C is a schematic diagram providing an exemplaryillustration of even mode currents that are inherent to the directionalcoupler 200 of FIG. 2A. Common elements between FIGS. 2A and 2C areshown therein with common element numbers and will not be re-describedherein.

As previously discussed in FIG. 2A, the signal 222 and the even modecurrent 236 flow in the same direction in the first microstrip 202 andthe second microstrip 204, respectively. In this regard, the signal 222and the even mode current 236 form the even mode currents in thedirectional coupler 200. The signal 222 flows along the first microstrip202 toward the output port (P2) 216 and then propagates through thesubstrate 208 to be sunk by the ground plane 210. The even mode current236 flows toward the isolated port (P4) 220 along the second microstrip204 and then propagates through the substrate 208 to be sunk by theground plane 210.

A capacitance C_(e) associated with the even mode currents can beequated with the capacitance of an even mode capacitance circuit 242.The even mode capacitance circuit 242 includes a first capacitor C11 anda second capacitor C22. The first capacitor C11 represents a firstcapacitance between the first microstrip 202 and the ground plane 210.The second capacitor C22 represents a second capacitance between thesecond microstrip 204 and the ground plane 210. As such, the capacitanceC_(e) associated with the even mode currents can be determined based onthe equation (Eq. 8) below.C _(e) =C11=C22  (Eq. 8)

FIG. 2D is a schematic diagram providing an exemplary illustration ofodd mode currents that are inherent to the directional coupler 200 ofFIG. 2A. Common elements between FIGS. 2A and 2D are shown therein withcommon element numbers and will not be re-described herein.

As previously discussed in FIG. 2A, the signal 222 and the odd modecurrent 240 flow in opposite directions in the first microstrip 202 andthe second microstrip 204, respectively. In this regard, the signal 222and the odd mode current 240 form the odd mode currents in thedirectional coupler 200. The signal 222 flows along the first microstrip202 toward the output port (P2) 216 and then propagates through thesubstrate 208 to be sunk by the ground plane 210. The odd mode current240, in contrast, is generated by the electrical field 238 and flowsfrom the isolated port (P4) 220 toward the coupled port (P3) 218 alongthe second microstrip 204.

A capacitance C_(o) associated with the odd mode currents can be equatedwith the capacitance of an odd mode capacitance circuit 244. The oddmode capacitance circuit 244 includes the first capacitor C11, thesecond capacitor C22, and a pair of third capacitors C12. The firstcapacitor C11 represents the first capacitance between the firstmicrostrip 202 and the ground plane 210. The second capacitor C22represents the second capacitance between the second microstrip 204 andthe ground plane 210. Each of the third capacitors C12 represents athird capacitance between the first microstrip 202 and the secondmicrostrip 204. As such, the capacitance C_(o) associated with the oddmode currents can be determined based on the equation (Eq. 9) below.C _(o) =C11+2C12=C22+2C12  (Eq. 9)

Based on Eq. 8 and Eq. 9 above, it can be noted that the odd modecapacitance C_(o) is higher than the even mode capacitance C_(e).Accordingly, it can be said that relative permittivity of thetransmission medium ε_(r) _(_) _(odd) for conveying the odd mode current240 is lower than relative permittivity of the transmission medium ε_(r)_(_) _(even) for conveying the even mode current 236. As such, it ispossible to determine even mode phase velocity ν_(even) of the even modecurrent 236 and odd mode phase velocity ν_(odd) of the odd mode current240 based on equation (Eq. 10) and equation (Eq. 11), respectively.

$\begin{matrix}{v_{even} = \frac{V_{light}}{\sqrt{ɛ_{r\;\_\;{even}}ɛ_{0}\mu_{r}\mu_{0}}}} & \left( {{Eq}.\mspace{14mu} 10} \right) \\{v_{odd} = \frac{V_{light}}{\sqrt{ɛ_{r\;\_\;{odd}}ɛ_{0}\mu_{r}\mu_{0}}}} & \left( {{Eq}.\mspace{14mu} 11} \right)\end{matrix}$

In Eq. 10 and Eq. 11 above, V_(light) represents the speed of light,which is approximately 187,000 miles per second, ε₀ represents vacuumpermittivity, and μ_(r)μ₀ would equal one when the first microstrip 202and the second microstrip 204 are provided as non-magnetic coppertraces. Notably, since the relative permittivity of the transmissionmedium ε_(r) _(_) _(odd) is lower than the relative permittivity of thetransmission medium ε_(r) _(_) _(even), the odd mode phase velocityν_(odd) of the odd mode current 240 would be higher than the even modephase velocity ν_(even) of the even mode current 236.

The difference between the odd mode phase velocity ν_(odd) and the evenmode phase velocity ν_(even) may be attributed to the fact that both thefirst microstrip 202 and the second microstrip 204 are onlyquasi-transverse electromagnetic (TEM), not fully TEM like coax orstripline mediums. According to discussions in FIGS. 2C-2D above, theeven mode current 236 flows along the second microstrip 204 toward theisolated port (P4) 220, propagates through the substrate 208, andsubsequently returns to the coupled port (P3) 218 though the groundplane 210. In contrast, the odd mode current 240 is induced by theelectrical field 238 and flows toward the coupled port (P3) 218 via thesecond microstrip 204. Notably, the even mode current 236 propagatesthrough the substrate 208 that has a higher permittivity ϵ_(sub) (e.g.,≈3.66 as in Rogers 4350B commercial substrate laminates). In contrast,the odd mode current 240 propagates through the electrical field 238that as a lower permittivity ε_(air) (≈2.8). Understandably, a portionof the odd mode current 240 may still propagate through the substrate208 and return to the coupled port (P3) 218 through the ground plane210. However, it is possible to significantly reduce the portion of theodd mode current 240 propagating through the substrate 208 by increasingthickness of the substrate 208 to twenty millimeters (20 mm), forexample. As a result, the odd mode current 240 may flow toward thecoupled port (P3) 218 primarily through the electrical field 238 and thesecond microstrip 204. In this regard, because the odd mode phasevelocity ν_(odd) of the odd mode current 240 is higher than the evenmode phase velocity ν_(even) of the even mode current 236, the odd modecurrent 240 and the even mode current 236 may not be destructivelycancelled. As a result, the directivity D of the directional coupler 200may lower than what is needed to ensure an accurate measurement of thecoupled signal 224 at the coupled port (P3) 218.

With reference back to FIG. 2A, the first microstrip 202 and the secondmicrostrip 204 in the directional coupler 200 each have a length 246 anda width 248. The length 246 may be approximately one quarter (¼) of acenter frequency wavelength λ of the directional coupler 200 and thewidth 248 may be so determined to provide 50Ω impedance.

In a non-limiting example, the bandwidth of the directional coupler 200may be three times the bandwidth of the signal 222 due tointermodulation products that may be associated with the signal 222. Assuch, another important figure of merit of the directional coupler 200relates to coupling ripples across the entire bandwidth of thedirectional coupler 200. In this regard, FIG. 2E is a graph 250providing an exemplary illustration of various coupling ripples in thedirectional coupler 200 of FIG. 2A.

With reference to FIG. 2E, in a non-limiting example, the directionalcoupler 200 of FIG. 2A receives the signal 222 at a center frequency of1960 MHz. The directional coupler 200 is configured to have a totalbandwidth of 300 MHz (1810-2110 MHz). The graph 250 thus illustrates thecoupling ripple associated with the directional coupler 200 across thetotal bandwidth of 300 MHz. The graph 250 includes a first curve 252that illustrates coupling ripples between the coupled port (P3) 218 andthe input port (P1) 214 of FIG. 2A. The graph 250 includes a secondcurve 254 that illustrates coupling ripples between the coupled port(P3) 218 and the output port (P2) 216 of FIG. 2A. According to the firstcurve 252, the coupling ripples between the coupled port (P3) 218 andthe input port (P1) 214 are below 0.2 dB. According to the second curve254, the coupling ripples between the coupled port (P3) 218 and theoutput port (P2) 216 are also below 0.2 dB. As such, the couplingripples of the directional coupler 200 are relatively low.

According to previous discussions in FIGS. 2A-2D, directivity D isanother important figure of merit of the directional coupler 200. Inthis regard, FIG. 2F is a graph 256 providing an exemplary illustrationof the directivity D of the directional coupler 200 of FIG. 2A. Like thegraph 250 of FIG. 2E, the graph 256 is also plotted across the 300 MHzbandwidth of the directional coupler 200. The graph 256 includes adirectivity curve 258, which illustrates that the directivity D is below3 dB. Notably, the 3 dB directivity D of the directional coupler 200 isrelatively low. As a result, according to previous discussions in FIGS.2C-2D, the odd mode current 240 and the even mode current 236 would notcancel out each other. Consequently, the coupled signal 224 may becomemore sensitive to impedance mismatch (reflection) at the output port(P2) 216, and an inaccurate measurement of the coupled signal 224 at thecoupled port (P3) 218 may result. There may be techniques to improve thedirectivity D from 3 dB to 7 dB, for example. However, a simulationperformed with the same 300 MHz bandwidth of the directional coupler 200indicates that the coupling ripple of the directional coupler 200 couldincrease from the previous 0.2 dB to approximately 0.9 dB. In thisregard, those existing techniques achieve improvement in the directivityD at the expense of the coupling ripple. Hence, it may be desired toimprove the directivity D of the directional coupler 200 withoutcompromising other performance measures, such as the coupling ripple.

In this regard, FIG. 3A is a schematic diagram of an exemplaryhigh-directivity directional coupler 300 that can provide improveddirectivity D without compromising other performance measures (e.g.,coupling ripple) in the high-directivity directional coupler 300. In theexemplary aspects discussed hereinafter, the high-directivitydirectional coupler 300 refers to a directional coupler capable ofproviding a directivity D in excess of 15 dB. The high-directivitydirectional coupler 300 is illustrated herein in a three-dimensional(3D) coordinate system 302 consisting of an x-axis 304, a y-axis 306,and a z-axis 308. Notably, orientation-related phrases, such as “above”and “underneath” are used hereinafter in relation to the z-axis 308.

With reference to FIG. 3A, the high-directivity directional coupler 300includes a substrate 310, which may be a printed circuit board (PCB) forexample. The high-directivity directional coupler 300 includes an inputport 312, an output port 314, a coupled port 316, and an isolated port318. The input port 312 is configured to receive an input signal 320.The output port 314 is configured to output the input signal 320 as anoutput signal 322. The coupled port 316 is configured to output acoupled signal 324 that is proportional to the input signal 320. Theisolated port 318 is configured to provide isolation to the input signal320 and the coupled signal 324. The high-directivity directional coupler300 includes a ground plane 326 that is disposed underneath thesubstrate 310. The high-directivity directional coupler 300 includes afirst microstrip 328 disposed above the substrate 310 and a secondmicrostrip 330 disposed above the substrate 310 parallel to the firstmicrostrip 328. The first microstrip 328 is configured to convey theinput signal 320 from the input port 312 to the output port 314.

According to previous discussions in FIGS. 2A and 2C-2D, when thehigh-directivity directional coupler 300 receives the input signal 320and outputs the output signal 322, the input signal 320 inherentlycauses an even mode current 332 and an odd mode current 334 in thesecond microstrip 330 as a result of the even mode excitation and theodd mode excitation, respectively. The even mode current 332 in thesecond microstrip 330, which flows in the same direction as the inputsignal 320 in the first microstrip 328, travels from the coupled port316 toward the isolated port 318 and then propagates through thesubstrate 310 to be sunk by the ground plane 326. The odd mode current334 travels in the opposite direction from the input signal 320 in thefirst microstrip 328 and the even mode current 332 in the secondmicrostrip 330.

As previously discussed, the odd mode current 334 has an odd mode phasevelocity ν_(odd) that is higher than an even mode phase velocityν_(even) of the even mode current 332. As such, for the high-directivitydirectional coupler 300 to render the directivity D in excess of 15 dB,it would be necessary to slow down the odd mode current 334 to match theodd mode phase velocity ν_(odd) with the even mode phase velocityν_(even). As is further discussed below, in the high-directivitydirectional coupler 300, the odd mode current 334 takes a longer returnpath to return to the coupled port 316. The longer return path causesthe odd mode current 334 to slow down. As a result, the odd mode phasevelocity ν_(odd) would match the even mode phase velocity ν_(even) andthe odd mode current 334 would be cancelled out by the even mode current332. Hence, the high-directivity directional coupler 300 can renderhigh-directivity without comprising other performance aspects (e.g.,coupling ripple) of the high-directivity directional coupler 300.

The second microstrip 330 provides a linear forward path 338 forconveying the even mode current 332 from the coupled port 316 to theground plane 326. In this regard, the even mode current 332 flows at theeven mode phase velocity ν_(even) along the second microstrip 330,propagates through the substrate 310, and returns to the coupled port316 via the ground plane 326. Given that the even mode current 332returns to the coupled port 316 primarily via the ground plane 326, theeven mode current 332 would travel along a shortest path with a lowestinductance. As such, the linear forward path 338 would be closer to anouter edge 340 of the second microstrip 330 and away from the firstmicrostrip 328.

The second microstrip 330 also provides a non-linear return path 342 forconveying the odd mode current 334 in an opposite direction from theeven mode current 332. The non-linear return path 342 is longer than thelinear forward path 338 to slow down the odd mode current 334 to causedestructive cancellation between the even mode current 332 and the oddmode current 334. In this regard, the non-linear return path 342 cancompensate for an inequality between the odd mode phase velocity ν_(odd)and the even mode phase velocity ν_(even). To provide the non-linearreturn path 342, the second microstrip 330 includes a plurality ofrectangular-shaped stubs 344 disposed adjacent to the first microstrip328. The rectangular-shaped stubs 344 and the non-linear return path 342formed by the rectangular-shaped stubs 344 are further discussed belowwith reference to FIG. 3B. In this regard, FIG. 3B is a schematicdiagram providing an exemplary top view 346 of the high-directivitydirectional coupler 300 of FIG. 3A. Common elements between FIGS. 3A and3B are shown therein with common element numbers and will not bere-described herein.

With reference to FIG. 3B, each of the rectangular-shaped stubs 344 isseparated from an adjacent rectangular-shaped stub among therectangular-shaped stubs 344 by a separation distance 348. Each of therectangular-shaped stubs 344 has a respective width 350. In this regard,in a non-limiting example, the rectangular-shaped stubs 344 can have anequal width 350 and are separated by an equal separation distance 348.In another non-limiting example, the respective width 350 is less thanor equal to sixty percent (60%) of the separation distance 348 to helpreduce potential parasitic capacitance between the rectangular-shapedstubs 344.

Given that the odd mode current 334 flows in the second microstrip 330in opposite direction from the even mode current 332 flowing in thefirst microstrip 328, the odd mode current 334 in the second microstrip330 would take a path with the least inductance to return to the coupledport 316. As such, as shown in FIG. 3B, the non-linear return path 342is formed along outer edges of the rectangular-shaped stubs 344 andadjacent to the first microstrip 328.

In one non-limiting example, a total length of the non-linear returnpath 342 can be determined based on a ratio between the odd mode phasevelocity ν_(odd) and the even mode phase velocity ν_(even). In anothernon-limiting, the total length of the non-linear return path 342 can bedetermined by connecting test equipment at the coupled port 316.Accordingly, the total length of the non-linear return path 342 can beadjusted continuously until a desirable reading of the coupled signal324 is obtained at the coupled port 316.

The high-directivity directional coupler 300 of FIG. 3A can befabricated based on a process. In this regard, FIG. 4 is a flowchart ofan exemplary process 400 for fabricating the high-directivitydirectional coupler 300 of FIG. 3A.

With reference to FIG. 4, the substrate 310 is first provided (block402). The ground plane 326 is then provided underneath the substrate 310(block 404). The input port 312 is provided to receive the input signal320 (block 406). The output port 314 is provided to output the receivedinput signal 320 as the output signal 322 (block 408). The coupled port316 is provided to output the coupled signal 324 proportional to theinput signal 320 (block 410). The isolated port 318 is provided toprovide isolation to the input signal 320 and the coupled signal 324(block 412). The first microstrip 328 is disposed above the substrate310 to convey the input signal 320 from the input port 312 to the outputport 314 (block 414). The second microstrip 330 is disposed above thesubstrate 310 parallel to the first microstrip 328 (block 416). Thesecond microstrip 330 provides the linear forward path 338 for conveyingthe even mode current 332 from the coupled port 316 to the ground plane326. The second microstrip 330 also provides the non-linear return path342 longer than the linear forward path 338 for conveying the odd modecurrent 334 in an opposite direction from the even mode current 332.

With reference back to FIG. 3A, the high-directivity directional coupler300 provides significantly higher directivity than the directionalcoupler 200 of FIG. 2A while maintaining a similar level of couplingripples as the directional coupler 200 of FIG. 2A. In this regard, FIG.5A is a graph 500 providing an exemplary illustration of variouscoupling ripples in the high-directivity directional coupler 300 of FIG.3A.

With reference to FIG. 5A, in a non-limiting example, thehigh-directivity directional coupler 300 of FIG. 3A receives the inputsignal 320 at a center frequency of 1960 MHz. The high-directivitydirectional coupler 300 is configured to have a total bandwidth of 300MHz (1810-2110 MHz). The graph 500 thus illustrates the coupling rippleassociated with the high-directivity directional coupler 300 across thetotal bandwidth of 300 MHz. The graph 500 includes a first curve 502that illustrates coupling ripples between the coupled port 316 and theinput port 312 of FIG. 3A. The graph 500 includes a second curve 504that illustrates coupling ripples between the coupled port 316 and theoutput port 314 of FIG. 3A. According to the first curve 502, thecoupling ripples between the coupled port 316 and the input port 312 arebelow 0.2 dB. According to the second curve 504, the coupling ripplesbetween the coupled port 316 and the output port 314 are also below 0.2dB. As such, the coupling ripples in the high-directivity directionalcoupler 300 are similar to the coupling ripples of the directionalcoupler 200 as shown in FIG. 2E.

FIG. 5B is a graph 506 providing an exemplary illustration of higherdirectivity in the high-directivity directional coupler 300 of FIG. 3Acompared to the directional coupler 200 of FIG. 2A. Like the graph 500of FIG. 3A, the graph 506 is also plotted across the 300 MHz bandwidthof the high-directivity directional coupler 300. The graph 506 includesa directivity curve 508, which illustrates that the directivity of thehigh-directivity directional coupler 300 is above 20 dB. Hence, thehigh-directivity directional coupler 300 can provide significantlyhigher directivity than the directional coupler 200 of FIG. 2A withoutincreasing coupling ripple, insertion loss, return loss, power handling,and price.

With reference back to FIG. 3A, in addition to providing higherdirectivity than the directional coupler 200 of FIG. 2A and maintainingsimilar coupling ripples as the directional coupler 200, thehigh-directivity directional coupler 300 is also more advantageous thanthe commercial low temperature co-fired ceramic (LTCC) (chip component)based directional coupler 200 in a variety of areas. In one aspect, thehigh-directivity directional coupler 300 can be simply printed on adistributed PCB as opposed to a high cost multilayer PCB. In anotheraspect, the high-directivity directional coupler 300 can support a wideroperational bandwidth, making it suitable for wideband RF applicationswith up to 500 MHz operational bandwidth. As is further discussed nextin FIG. 6, the high-directivity directional coupler 300 can also be usedin applications employing digital pre-distortion (DPD) across widebandwidth. In another aspect, the high-directivity directional coupler300 only requires a 50Ω resistor on the isolated port 318. This isbecause the high-directivity directional coupler 300 does not need towithstand relatively higher power levels injected to the input port 312and coupled to the isolated port 318 given the ability of thehigh-directivity directional coupler 300 in cancelling out the even modecurrent 332 and the odd mode current 334. In another aspect, thehigh-directivity directional coupler 300 has very low insertion loss.This is due to the fact that insertion loss is a function of the firstmicrostrip 328 between the input port 312 and the output port 314. In anon-limiting example, the first microstrip 328 has a respective width352 that renders the first microstrip with 50Ω resistance. As such, theinsertion loss of the high-directivity directional coupler 300 isminimal. In another aspect, the high-directivity directional coupler 300has very high return loss. This is due to the fact that return loss is afunction of the first microstrip 328 between the input port 312 and theoutput port 314. As such, the 50Ω resistance of the first microstrip 328can maximize the return loss. On the other hand, return loss at thecoupled port 316 is a function of the 50Ω resistor coupled to theisolated port 318. As such, the return loss at the coupled port 316 ishigh as well. In another aspect, both the first microstrip 328 and thesecond microstrip 330 can withstand very high power, thus making thehigh-directivity directional coupler 300 suitable for carrying higherpower (e.g., class 3) RF signals. In another aspect, thehigh-directivity directional coupler 300 can be manufactured with lowercosts and a smaller footprint compared to the directional coupler 200 ofFIG. 2A.

As mentioned earlier, the high-directivity directional coupler 300 canbe used in applications employing DPD across wide bandwidth. In thisregard, FIG. 6 is a schematic diagram of an exemplary RF powermanagement circuit 600 in which the high-directivity directional coupler300 of FIG. 3A is configured to provide an RF feedback signal 602 from apower amplifier 604 to a DPD circuit 606 to help improve linearity ofthe power amplifier 604. Common elements between FIGS. 3A and 6 areshown therein with common element numbers and will not be re-describedherein.

With reference to FIG. 6, the power amplifier 604 is configured toreceive and amplify an RF signal 608 and provide the amplified RF signal608 to the input port 312 of the high-directivity directional coupler300. The high-directivity directional coupler 300 is coupled to anantenna 610 via the output port 314. The high-directivity directionalcoupler 300 is configured to output the amplified RF signal 608 to theantenna 610 for transmission to client devices.

The DPD circuit 606 is configured to receive a digital signal 612 andprovide linearity correction to the digital signal 612 based on adigital feedback signal 614 received via a feedback path 616. The RFpower management circuit 600 includes a digital-to-analog converter(DAC) 618 configured to convert the digital signal 612 into the RFsignal 608. The RF power management circuit 600 includes a frequencyup-converter 620 configured to convert the RF signal 608 from a basebandfrequency to a carrier frequency and provide the RF signal 608 to thepower amplifier 604.

The feedback path 616 is configured to receive the RF feedback signal602 via the coupled port 316 of the high-directivity directional coupler300. The feedback path 616 includes an attenuator 622 configured toreduce the power of the RF feedback signal 602. The feedback path 616includes a frequency down-converter 624 configured to convert the RFfeedback signal 602 from the carrier frequency to the basebandfrequency. The feedback path 616 also includes an analog-to-digitalconverter (ADC) 626 to convert the RF feedback signal 602 to the digitalfeedback signal 614 and provide the digital feedback signal 614 to theDPD circuit 606.

According to earlier discussions in FIG. 3A, the high-directivitydirectional coupler 300 can provide higher directivity in combinationwith lower coupling ripple and insertion loss. As such, thehigh-directivity directional coupler 300 can provide accurate feedbackabout actual power level outputted by the power amplifier 604.Accordingly, the DPD circuit 606 can effectively pre-distort the digitalsignal 612 to improve linearity in the power amplifier 604.

The RF power management circuit 600 can be provided in a remote unit(s)in a wireless distribution system (WDS) to help fine tune a poweramplifier(s) in the remote unit. In this regard, FIG. 7 is a schematicdiagram an exemplary WDS 700 provided in the form of an opticalfiber-based WDS in which at least one selected remote unit among aplurality of remote units can employ the RF power management circuit 600of FIG. 6 for improving linearity of a power amplifier in the selectedremote unit.

The WDS 700 includes an optical fiber for distributing communicationsservices for multiple frequency bands. The WDS 700 in this example iscomprised of three (3) main components in this example. A plurality ofradio interfaces provided in the form of radio interface modules (RIMs)702(1)-702(M) are provided in a central unit 704 to receive and processa plurality of downlink digital communications signals 706D(1)-706D(R)prior to optical conversion into downlink optical fiber-basedcommunications signals. The downlink digital communications signals706D(1)-706D(R) may be received from a base station as an example. TheRIMs 702(1)-702(M) provide both downlink and uplink interfaces forsignal processing. The notations “1-R” and “1-M” indicate that anynumber of the referenced component, 1-R and 1-M, respectively, may beprovided. The central unit 704 is configured to accept the RIMs702(1)-702(M) as modular components that can easily be installed andremoved or replaced in the central unit 704. In one example, the centralunit 704 is configured to support up to twelve (12) RIMs 702(1)-702(12).Each RIM 702(1)-702(M) can be designed to support a particular type ofradio source or range of radio sources (i.e., frequencies) to provideflexibility in configuring the central unit 704 and the WDS 700 tosupport the desired radio sources.

For example, one RIM 702 may be configured to support the PersonalizedCommunications System (PCS) radio band. Another RIM 702 may beconfigured to support the 800 megahertz (MHz) radio band. In thisexample, by inclusion of the RIMs 702(1)-702(M), the central unit 704could be configured to support and distribute communications signals onboth PCS and Long-Term Evolution (LTE) 700 radio bands, as an example.The RIMs 702(1)-702(M) may be provided in the central unit 704 thatsupport any frequency bands desired, including but not limited to the USCellular band, PCS band, Advanced Wireless Service (AWS) band, 700 MHzband, Global System for Mobile communications (GSM) 900, GSM 1800, andUniversal Mobile Telecommunications System (UMTS). The RIMs702(1)-702(M) may also be provided in the central unit 704 that supportany wireless technologies desired, including but not limited to CodeDivision Multiple Access (CDMA), CDMA200, 1×RTT, Evolution—Data Only(EV-DO), UMTS, High-speed Packet Access (HSPA), GSM, General PacketRadio Services (GPRS), Enhanced Data GSM Environment (EDGE), TimeDivision Multiple Access (TDMA), LTE, iDEN, and Cellular Digital PacketData (CDPD).

The RIMs 702(1)-702(M) may be provided in the central unit 704 thatsupport any frequencies desired, including but not limited to US FCC andIndustry Canada frequencies (824-849 MHz on uplink and 869-894 MHz ondownlink), US FCC and Industry Canada frequencies (1850-1915 MHz onuplink and 1930-1995 MHz on downlink), US FCC and Industry Canadafrequencies (1710-1755 MHz on uplink and 2110-2155 MHz on downlink), USFCC frequencies (698-716 MHz and 776-787 MHz on uplink and 728-746 MHzon downlink), EU R & TTE frequencies (880-915 MHz on uplink and 925-960MHz on downlink), EU R & TTE frequencies (1710-1785 MHz on uplink and1805-1880 MHz on downlink), EU R & TTE frequencies (1920-1980 MHz onuplink and 2110-2170 MHz on downlink), US FCC frequencies (806-824 MHzon uplink and 851-869 MHz on downlink), US FCC frequencies (896-901 MHzon uplink and 929-941 MHz on downlink), US FCC frequencies (793-805 MHzon uplink and 763-775 MHz on downlink), and US FCC frequencies(2495-2690 MHz on uplink and downlink).

With continuing reference to FIG. 7, the downlink digital communicationssignals 706D(1)-706D(R) are provided to a plurality of opticalinterfaces provided in the form of optical interface modules (OIMs)708(1)-708(N) in this embodiment to convert the downlink digitalcommunications signals 706D(1)-706D(R) into a plurality of downlinkoptical fiber-based communications signals 710D(1)-710D(R). The notation“1-N” indicates that any number of the referenced component 1-N may beprovided. The OIMs 708(1)-708(N) may be configured to provide aplurality of optical interface components (OICs) that containoptical-to-electrical (O/E) and electrical-to-optical (E/O) converters,as will be described in more detail below. The OIMs 708(1)-708(N)support the radio bands that can be provided by the RIMs 702(1)-702(M),including the examples previously described above.

The OIMs 708(1)-708(N) each include E/O converters to convert thedownlink digital communications signals 706D(1)-706D(R) into thedownlink optical fiber-based communications signals 710D(1)-710D(R). Thedownlink optical fiber-based communications signals 710D(1)-710D(R) arecommunicated over a downlink optical fiber-based communications medium712D to a plurality of remote units 714(1)-714(S). At least one selectedremote unit among the remote units 714(1)-714(S), for example the remoteunit 714(1), can include the RF power management circuit 600 of FIG. 6for improving linearity of a power amplifier in the selected remoteunit. The notation “1-S” indicates that any number of the referencedcomponent 1-S may be provided. Remote unit O/E converters provided inthe remote units 714(1)-714(S) convert the downlink optical fiber-basedcommunications signals 710D(1)-710D(R) back into the downlink digitalcommunications signals 706D(1)-706D(R), which are the converted into aplurality of downlink RF communications signals 715D(1)-715D(S) andprovided to antennas 716(1)-716(S) in the remote units 714(1)-714(S) toclient devices in the reception range of the antennas 716(1)-716(S).

In this regard, in the selected remote unit 714(1), the DPD circuit 606of FIG. 6 receives a selected downlink digital communications signalamong the downlink digital communications signals 706D(1)-706D(R). TheDAC 618 converts the selected downlink digital communications signalinto a selected downlink RF communications signal among the downlink RFcommunications signals 715D(1)-715D(S). The frequency up-converter 620converts the selected downlink RF communications signal from a basebandfrequency to a carrier frequency. The power amplifier 604 amplifies theselected downlink RF communications signal. The high-directivitydirectional coupler 300 receives and provides the downlink RFcommunications signal to the antenna 610 for distribution to the clientdevices.

The remote units 714(1)-714(S) receive a plurality of uplink RFcommunications signals 715U(1)-715U(S) from the client devices throughthe antennas 716(1)-716(S). The remote units 714(1)-714(S) covert theuplink RF communications signals 715U(1)-715U(S) into a plurality ofuplink digital communications signals. Remote unit E/O converters arealso provided in the remote units 714(1)-714(S) to convert the uplinkdigital communications signals into a plurality of uplink opticalfiber-based communications signals 710U(1)-710U(S). The remote units714(1)-714(S) communicate the uplink optical fiber-based communicationssignals 710U(1)-710U(S) over an uplink optical fiber-basedcommunications medium 712U to the OIMs 708(1)-708(N) in the central unit704. The OIMs 708(1)-708(N) include O/E converters that convert thereceived uplink optical fiber-based communications signals710U(1)-710U(S) into uplink digital communications signals720U(1)-720U(S), which are processed by the RIMs 702(1)-702(M) andprovided as the uplink digital communications signals 720U(1)-720U(S).The central unit 704 may provide the uplink digital communicationssignals 720U(1)-720U(S) to a base station or other communicationssystem.

Note that the downlink optical fiber-based communications medium 712Dand the uplink optical fiber-based communications medium 712U connectedto each of the remote units 714(1)-714(S) may be a common opticalfiber-based communications medium, wherein for example, wave divisionmultiplexing (WDM) is employed to provide the downlink opticalfiber-based communications signals 710D(1)-710D(R) and the uplinkoptical fiber-based communications signals 710U(1)-710U(S) on the sameoptical fiber-based communications medium.

The WDS 700 of FIG. 7 may be provided in an indoor environment, asillustrated in FIG. 8. FIG. 8 is a partial schematic cut-away diagram ofan exemplary building infrastructure 800 in which a WDS, such as the WDS700 of FIG. 7, includes a remote unit that employs the high-directivitydirectional coupler 300 of FIG. 3A to provide linearity improvement in apower amplifier in the remote unit. The building infrastructure 800 inthis embodiment includes a first (ground) floor 802(1), a second floor802(2), and a third floor 802(3). The floors 802(1)-802(3) are servicedby a central unit 804 to provide antenna coverage areas 806 in thebuilding infrastructure 800. The central unit 804 is communicativelycoupled to a base station 808 to receive downlink communications signals810D from the base station 808. The central unit 804 is communicativelycoupled to a plurality of remote units 812 to distribute the downlinkcommunications signals 810D to the remote units 812 and to receiveuplink communications signals 810U from the remote units 812, aspreviously discussed above. The downlink communications signals 810D andthe uplink communications signals 810U communicated between the centralunit 804 and the remote units 812 are carried over a riser cable 814.The riser cable 814 may be routed through interconnect units (ICUs)816(1)-816(3) dedicated to each of the floors 802(1)-802(3) that routethe downlink communications signals 810D and the uplink communicationssignals 810U to the remote units 812 and also provide power to theremote units 812 via array cables 818.

The embodiments disclosed herein include various steps. The steps of theembodiments disclosed herein may be formed by hardware components or maybe embodied in machine-executable instructions, which may be used tocause a general-purpose or special-purpose processor programmed with theinstructions to perform the steps. Alternatively, the steps may beperformed by a combination of hardware and software.

The embodiments disclosed herein may be provided as a computer programproduct, or software, that may include a machine-readable medium (orcomputer-readable medium) having stored thereon instructions, which maybe used to program a computer system (or other electronic devices) toperform a process according to the embodiments disclosed herein. Amachine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes: amachine-readable storage medium (e.g., ROM, random access memory(“RAM”), a magnetic disk storage medium, an optical storage medium,flash memory devices, etc.); and the like.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps, or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is in no way intendedthat any particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications, combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A high-directivity directional coupler,comprising: a substrate; a ground plane disposed underneath thesubstrate; an input port configured to receive an input signal; anoutput port configured to output the received input signal as an outputsignal; a coupled port configured to output a coupled signalproportional to the input signal; an isolated port configured to provideisolation to the input signal and the coupled signal; a first microstripdisposed above the substrate, the first microstrip configured to conveythe input signal from the input port to the output port; and a secondmicrostrip disposed above the substrate parallel to the firstmicrostrip, the second microstrip configured to: provide a linearforward path for conveying an even mode current from the coupled port tothe ground plane; and provide a non-linear return path longer than thelinear forward path for conveying an odd mode current in an oppositedirection from the even mode current.
 2. The high-directivitydirectional coupler of claim 1, wherein the non-linear return path isconfigured to compensate for an inequality between an odd mode phasevelocity of the odd mode current and an even mode phase velocity of theeven mode current.
 3. The high-directivity directional coupler of claim2, wherein the non-linear return path is further configured to delay theodd mode current to cause the odd mode current to be substantiallycancelled out by the even mode current.
 4. The high-directivitydirectional coupler of claim 1, wherein the second microstrip comprisesa plurality of rectangular-shaped stubs disposed adjacent to the firstmicrostrip, each of the plurality of rectangular-shaped stubs isseparated from an adjacent rectangular-shaped stub among the pluralityof rectangular-shaped stubs by a separation distance.
 5. Thehigh-directivity directional coupler of claim 4, wherein each of theplurality of rectangular-shaped stubs has a respective width that isless than or equal to sixty percent of the separation distance.
 6. Thehigh-directivity directional coupler of claim 4, wherein the non-linearreturn path is formed along outer edges of the plurality ofrectangular-shaped stubs.
 7. A method for fabricating a high-directivitydirectional coupler, comprising: providing a substrate; providing aground plane underneath the substrate; providing an input portconfigured to receive an input signal; providing an output portconfigured to output the received input signal as an output signal;providing a coupled port configured to output a coupled signalproportional to the input signal; providing an isolated port configuredto provide isolation to the input signal and the coupled signal;disposing a first microstrip above the substrate, the first microstripconfigured to convey the input signal from the input port to the outputport; and disposing a second microstrip above the substrate parallel tothe first microstrip, the second microstrip configured to: provide alinear forward path for conveying an even mode current from the coupledport to the ground plane; and provide a non-linear return path longerthan the linear forward path for conveying an odd mode current in anopposite direction from the even mode current.
 8. The method of claim 7,further comprising compensating for an inequality between an odd modephase velocity of the odd mode current and an even mode phase velocityof the even mode current.
 9. The method of claim 8, further comprisingdelaying the odd mode current to cause the odd mode current to besubstantially cancelled out by the even mode current.
 10. The method ofclaim 7, further comprising: disposing a plurality of rectangular-shapedstubs on the second microstrip adjacent to the first microstrip; andseparating each rectangular-shaped stub from an adjacentrectangular-shaped stub among the plurality of rectangular-shaped stubsby a separation distance.
 11. A wireless distribution system (WDS),comprising: a plurality of remote units configured to: receive andconvert a plurality of downlink digital communications signals into aplurality of downlink radio frequency (RF) communications signals fordistribution to client devices; and receive a plurality of uplink RFcommunications signals from the client devices and convert the pluralityof uplink RF communications signals into a plurality of uplink digitalcommunications signals; and a central unit configured to: distribute theplurality of downlink digital communications signals to the plurality ofremote units; and receive the plurality of uplink digital communicationssignals from the plurality of remote units; wherein at least oneselected remote unit among the plurality of remote units comprises ahigh-directivity directional coupler, comprising: a substrate; a groundplane disposed underneath the substrate; an input port configured toreceive an input signal; an output port configured to output thereceived input signal as an output signal; a coupled port configured tooutput a coupled signal proportional to the input signal; an isolatedport configured to provide isolation to the input signal and the coupledsignal; a first microstrip disposed above the substrate, the firstmicrostrip configured to convey the input signal from the input port tothe output port; and a second microstrip disposed above the substrateparallel to the first microstrip, the second microstrip configured to:provide a linear forward path for conveying an even mode current fromthe coupled port to the ground plane; and provide a non-linear returnpath longer than the linear forward path for conveying an odd modecurrent in an opposite direction from the even mode current.
 12. The WDSof claim 11, wherein the at least one selected remote unit furthercomprises: a power amplifier coupled to the input port of thehigh-directivity directional coupler and configured to amplify aselected downlink RF communications signal among the plurality ofdownlink RF communications signals; and an antenna coupled to the outputport of the high-directivity directional coupler and configured todistribute the selected downlink RF communications signal to the clientdevices.
 13. The WDS of claim 12, wherein the at least one selectedremote unit further comprises: a digital pre-distortion (DPD) circuitconfigured to provide linearity correction in a selected downlinkdigital communications signal among the plurality of downlink digitalcommunications signals based on a digital feedback signal received via afeedback path; a digital-to-analog converter (DAC) configured to convertthe selected downlink digital communications signal into the selecteddownlink RF communications signal; and a frequency up-converterconfigured to convert the selected downlink RF communications signalfrom a baseband frequency to a carrier frequency and provide theselected downlink RF communications signal to the power amplifier. 14.The WDS of claim 13, wherein the feedback path is configured to receivean RF feedback signal via the coupled port of the high-directivitydirectional coupler.
 15. The WDS of claim 14, wherein the feedback pathcomprises: a frequency down-converter configured to convert the RFfeedback signal from the carrier frequency to the baseband frequency;and an analog-to-digital converter (ADC) configured to convert the RFfeedback signal to the digital feedback signal and provide the digitalfeedback signal to the DPD circuit.
 16. The WDS of claim 12, wherein thenon-linear return path is further configured to delay the odd modecurrent to cause the odd mode current to be substantially cancelled outby the even mode current.
 17. The WDS of claim 11, wherein thenon-linear return path is configured to compensate for an inequalitybetween an odd mode phase velocity of the odd mode current and an evenmode phase velocity of the even mode current.
 18. The WDS of claim 11,wherein the second microstrip comprises a plurality ofrectangular-shaped stubs disposed adjacent to the first microstrip, eachof the plurality of rectangular-shaped stubs is separated from anadjacent rectangular-shaped stub among the plurality ofrectangular-shaped stubs by a separation distance.
 19. The WDS of claim18, wherein each of the plurality of rectangular-shaped stubs has arespective width that is less than or equal to sixty percent of theseparation distance.
 20. The WDS of claim 19, wherein the non-linearreturn path is formed along outer edges of the plurality ofrectangular-shaped stubs.
 21. The WDS of claim 11, wherein: the centralunit comprises electrical-to-optical (E/O) converters configured toconvert the plurality of downlink digital communications signals into aplurality of downlink optical fiber-based communications signals fordistribution to the plurality of remote units over a downlink opticalfiber-based communications medium; the plurality of remote unitscomprises: remote unit optical-to-electrical (O/E) converters configuredto convert the plurality of downlink optical fiber-based communicationssignals into the plurality of downlink digital communications signals;and remote unit E/O converters configured to convert the plurality ofuplink digital communications signals into a plurality of uplink opticalfiber-based communications signals; and the central unit furthercomprises O/E converters configured to convert the plurality of uplinkoptical fiber-based communications signals received from the pluralityof remote units over an uplink optical fiber-based communications mediuminto the plurality of uplink digital communications signals.